Rajesh, nanodiamonds itself should contain NV centers at least (most of them). Many of this nanodiamonds are commercially available (adamas nanotechnology, microdiamant etc.). But if you want to functionalize your own nanodiamonds, you can irradiate them with electron or helium beams at 10kev-10mev with 10^10-10^16 density and then anneal at 800-100 degC. If this doen't help you can implant Nitrogen Ions also, and then make the electron radiation and annealing.

But note that a size of ND for NV center to be stable is 15 nm or more, otherwise surface effects could affect it's fluorescence.